Epidemics were a key factor in the rise of Islam.
The Byzantine and Persian Empires had been ravaged by plague, especially the cities with their highly educated specialists. Their economies went into depression, and attention turned inward, leaving the door open for the growing influence of raiding tribes from the hinterlands.
Historically, epidemics are one of the recurring factors responsible for bringing down great societies - bankruptcy, overwhelming immigration, civil war, and military defeat - seems like we have moved toward higher risk for all of them.
We've had our own Black Plague since November 2008.